1. Field of the Invention
The present invention relates generally to the field of integrated circuit devices and more particularly to packages for integrated circuits where the package includes a heat sink. The present invention also relates to a method of making a package for integrated circuits including a method of forming a heat dissipation device.
2. Description of the Related Art
The performance levels of microelectronic devices, for example integrated circuits, power amplifiers, etc., are continually increasing to keep pace with the demands of modern technology. Performance levels such as clock speed are closely tied to the number and density of features, for example transistors, patterned onto the microelectronic device. Faster processing by the microelectronic device demands faster clock speeds. Faster clock speeds, in turn, mean more switching and power dissipation per unit time. The performance levels of microelectronic devices should continually improve as the size of the transistors is decreased and the density of the features is increased. However, small, closely packed features dissipate large amounts of heat which may limit performance levels. Heat is often dissipated from small, select regions of the device typically by heat sinks.
Temperature control has thus emerged as a limiting factor in the design of microelectronic devices. New-age devices, such as high-power amplifiers and multi-chip modules, radiate particularly large amounts of heat. Failure to effectively conduct heat away generally leaves these devices operating at high temperatures, ultimately resulting in decreased performance and reliability. This effect can be readily seen in semiconductor packages.
Semiconductor packages that include stacked semiconductor dies, otherwise known as semiconductor “chips” or “integrated circuits,” are becoming increasingly popular. Such packages allow dies that perform the same function, such as two memory dies, or different functions, such as a processor die and a memory die, to be combined into a single package. This improves density and is especially useful in applications where package size, particularly small package size, is important, such as in cell phones, PDAs, camcorders, and other wireless consumer products.
Moreover, an increase in the operation speed of integrated circuit devices is also rapidly advancing. With an increased integration density of integrated circuit devices the amount of heat generated correspondingly increases. Additionally, increased demand for downsizing modules accommodating microelectronic components including integrated circuit devices complicates both the need to remove excessive heat generated by integrated circuit devices and how to accomplish heat removal within an increasingly limited area. Thus the overall heat density associated with microelectronic devices and modules increases. Furthermore, many integrated circuit devices including memory devices and chip type circuit components are mounted on a printed circuit board so that the number of components mounted on a module has a tendency to increase and resulting in decreased surface area available for conventional heat sinks.
Integrated circuit devices are conventionally bundled together in packages which are then mounted on a printed circuit board to form multi-chip modules. Integrated circuit packages usually require a cover cap or encapsulant of some type over the integrated circuit devices to protect it and to provide a large flat surface for pick and place up operations. However, any cover cap or encapsulant above the integrated circuit increases the thermal resistance path to an ambient environment and hence the operation temperature of the integrated circuit, and the resulting heat density of the multi-chip module.
Various means are being used to mitigate the effects of such covers or caps and in general the high amount of heat generated within a package. One approach has been to use a capped integrated circuit with a thin layer of thermally conductive grease between the integrated circuit and the cap. Another means to decrease the operation temperature of the integrated circuit is to attach a heat sink on top of the cap or on top of the encapsulant. An example would be to attach a heat spreader with fins on top of the package, that is a heat spreader placed on top of the encapsulant or cap.
Another means to mitigate the effect of such covers or caps and generally decrease the overall operating temperature of the integrated circuits in the package are to include an internal heat spreader within the package. Another conventional means of heat dissipation include a heat sink coupled to the upper surface area of a die stack and to the surface area of the substrate on which the die stack lies. This design aims to improve the heat dissipation pathway that travels from the upper surface area of a die stack and over the substrate to the solder binding and to a printed circuit board. However, this design is successful mainly when the printed circuit board is adequately cool when compared to the package. Still, the conventional pathway for heat transfer is through the upper most surface of the package and from there into the ambient surroundings or even to another heat sink that is mounted on top of the package.
Nevertheless, one problem with the preferential heat dissipation pathway travelling in the vertical direction through the package, especially in die stack packages, is that the top most integrated circuit will be cooled much better than any underlying integrated circuits. One reason for this may be because the thermal resistance of the package from the bottom to the top surface accounts for a considerable part of the overall thermal resistance to the ambient surroundings. The thermal resistance is considerable because the heat must pass through the entire stack in order for any heat generated within the die stack to be discharged into the ambient atmosphere.
This means that for any generated heat to eventually dissipate through the top surface of a die stack, the heat must pass through multiple layers of various components to reach the upper most layer in the package. For instance, the heat discharge pathway may include going through the integrated circuits within the die stack, through any adhesive layers between the integrated circuits, and often through a spacer. Of course such a preferential heat dissipation pathway means that in order to cool the lower most integrated circuit within a stack, the heat must travel through the entire stack and any intermediate layers to be discharged through the upper surface and into the ambient surroundings, likely resulting in particular difficulties for sufficiently cooling the lower most integrated circuit within a stack.
Another potential drawback of having the heat transfer pathway preferentially travel from the bottom most integrated circuit through the die stack and to the top most integrated circuit is that each succeeding upper integrated circuit may be heated up by the lower integrated circuit as heat passes from a lower integrated circuit to an above lying integrated circuit and so on up the die stack. Moreover, there are some uses of stacked integrated circuits which do not permit the upper most surface to function simultaneously as a heat dissipation surface available for mounting a heat spreader. For example a sensor array type integrated circuit may not permit a heat sink attached to the upper surface of an integrated circuit package.
In most cases a heat sink is conventionally mounted on an integrated circuit package parallel to the upper integrated circuit surface area of the die stack. Such a heat sink serves the purpose of spreading the heat in a horizontal plane that is generally parallel to the upper integrated circuit surface area, resulting in the preferential vertical heat pathway through each die stack component. Furthermore, any intermediate interfaces between integrated circuits often contribute to the overall thermal resistance of a die stack thereby limiting the heat dissipation potential of the bottom most integrated circuit and so on through the stack up to the top most integrated circuit. Some examples of intermediate interfaces that may increase the heat resistance of a die stack include spacers or adhesives.